1. Field of the Invention
The present invention relates to a process for producing a thermoelectric material and to the thermoelectric material obtainable by said process.
More precisely, this invention is related to the fabrication of a thermoelectric material with outstanding properties, and to a manufacturing process thereof, which is superior to existing processes from an industrial standpoint.
The present invention further relates to the use of the thermoelectric material with outstanding properties obtainable by the process of the invention.
2. Description of the Prior Art
Thermoelectric materials based on the bismuth telluride, antimony telluride, bismuth selenide and antimony selenide family of alloys are currently widely used in a number of niche applications. These include cooling and temperature stabilization of electronic devices, as well as thermoelectric power generation. The working principle of devices made out of thermoelectric materials is related to the Peltier effect, and the closely related Seebeck effect. The thermoelectric device can either function as a heat pump when an electric current is allowed to flow (Peltier effect), or as a power source when a temperature gradient is imposed (Seebeck effect).
Investigated with significant resources in the early 60""s, thermoelectric materials are again being examined with scrutiny for large-scale applications, being viewed as a replacement technology for the ozone-depleting compressor technology, and a potential source of electric power generated from the recovery of waste heat. The main hurdle to be overcome remains the cost to performance ratio, which is still too high. This is the result of the intrinsically poor performance of the available thermoelectric materials, which is accompanied by an extreme brittleness, which makes manufacturing tedious and thermoelectric device lifetime difficult to predict.
There have recently been a number of attempts to overcome this problem. For example, a series of powder processing methods have recently been described in U.S. Pat. No. 5,448,109, U.S. Pat. No. 5,108,515, U.S. Pat. No. 5,318,743 and in U.S. Pat. No. 5,246,504 in which the thermoelectric material is pulverized and then typically pressed and sintered. The mechanical properties are generally improved in these instances but the thermoelectric properties are not, or if so, only marginally. For example, in the method described in U.S. Pat. No. 5,448,109, only the p-type leg is pressed and sintered and the n-type leg is produced using conventional techniques, its performance being otherwise too severally degraded. In addition, these methods involve additional processing steps (over current manufacturing practices), such as crushing, often using a solvent, pressing into a green billet and in all cases a final sintering step, which limits the attainable part size. Such methods are therefore not considered of practical interest.
Methods aimed at improving the thermoelectric properties and also based on powder processing routes have also been described in U.S. Pat. No. 4,588,520, U.S. Pat. No. 5,726,381, U.S. Pat. No. 5,763,293, U.S. Pat. No. 5,981,863 and in U.S. Pat. No. 6,043,424. In some of these a second phase is promoted, whether this be through the direct incorporation of dopants, as described in U.S. Pat. No. 4,588,520 or in U.S. Pat. No. 5,726,381 or indirectly through a special quenching technique as described in U.S. Pat. No. 5,763,293 and in U.S. Pat. No. 5,981,863. Also reported in U.S. Pat. No. 6,043,424 is an improvement of thermoelectric properties through the control of the degree of oxidation of grain boundaries. In the method described in U.S. Pat. No. 4,588,520,the improvement is noticeable only at elevated temperatures and is therefore not of general interest. In the patents U.S. Pat. No. 5,726,381, U.S. Pat. No. 5,763,293, U.S. Pat. No. 5,981,863 and in U.S. Pat. No. 6,043,424, the improvements reported for the thermoelectric materials thereby obtained appear to be difficult to reproduce and these methods are thus considered problematic. They are also of limited practical interest because of their complexity involving, as those previously described, various crushing, pressing and sintering steps.
The method described in patent application DE-A-1 197 34 471 is of practical interest since enabling the fabrication of net shape alloy using a hot extrusion technique. There is therefore no need to perform a final sintering step and it appears that this technique can be applied to both a powdered material or an otherwise consolidated material (bulk ingot, green billet). An improvement in both the mechanical and thermoelectric properties is reported, but the method does not lead to the production of large pieces. In some other work i.e in Fabrication and Characterization of Bi2Te3xe2x80x94Sb2Te3 based Thermoelectric Materials by Powder-Extrusion-Sintering Technique, 16th International Conference on Thermoelectrics (1997), p.76-80,by B-G. Min and al and in Effect of Extrusion process on the Thermoelectric Properties of Hot-Extruded N-type Bi2Te2.85Se0.15, 16th International Conference on Thermoelectrics (1997), p.81-84, extrusion of larger parts was attempted but macroscopic defects were then encountered to such an extent that the parts produced were of no practical interest.
Therefore, at this time, there is no method which would enable large parts exhibiting superior thermoelectric and mechanical properties to be produced. Furthermore, such large parts have never been reported. In addition, there is no known method, which would enable such large parts, if they could be obtained, to be produced using an efficient manufacturing process which could be scaleable leading to the large scale use of thermoelectric materials based on the bismuth telluride, antimony telluride, bismuth selenide and antimony selenide family of alloys.
More particularly, the present invention provides a process for producing a thermoelectric material based on two or more elements selected in the group constituted by Bi, Sb, Te and Se, which process comprises a mechanical alloying step wherein determined amounts of the elements Bi, Sb, Te or Se are mixed so as to obtain a powdered mechanical alloy. In accordance with the present invention, the powdered mechanical alloy may be a homogenous powdered mechanical alloy.
The present invention more specifically relates to a process for producing a thermoelectric material in the form of an extrudate by alloying, preferably by mechanically mixing determined amounts of the constituting elements of the thermoelectric material, which is preferably in a powdered form and by extruding the alloy thereby obtained with an extruder, which is preferably equipped with a multi-step die. The present invention further provides new extrudates of thermoelectric material based on two or more elements selected in the group constituted by Bi, Sb, Te and Se, which new extrudates being obtainable by the new process according to the invention. The extrudate thereby obtained, and in particular the high dimensional extrudates obtained by the process of the invention are useful in a suitable size and form, in devices with thermoelectric properties, particularly as cooler, as stabilizer of electronic devices, or as power generator.
More particularly, the different aspects of the invention may be summarized as follows.
In accordance with one aspect, the present invention relates to a process for producing a thermoelectric material based on two or more elements selected in the group constituted by Bi, Sb, Te and Se, which process may conprise:
i. a mechanical alloying step wherein determined amounts of the elements Bi, Sb, Te or Se are mixed so as to obtain a homogenous powdered mechanical alloy;
ii. an extrusion step, wherein said homogenous powdered mechanical alloy obtained in the preceding step is extruded.
In accordance with another aspect the present invention relates to a process for producing thermoelectric materials based on two or more elements selected in the group constituted by Bi, Sb, Te and Se, which process may comprise:
i. a mechanical alloying step wherein determined amounts of the elements Bi, Sb, Te or Se are mixed so as to obtain a homogenous powdered mechanical alloy;
ii. a compaction step, wherein said homogenous powdered mechanical alloy obtained in the preceding step is compacted; and
iii. an extrusion step, wherein the compacted alloy obtained in the preceding step is extruded.
In accordance with still another aspect, the present invention provides a process for producing thermoelectric materials based on two or more elements selected in the group constituted by Bi, Sb, Te and Se, which process may comprise:
i. a mechanical alloying step wherein determined amounts of the elements Bi, Sb, Te or Se are mixed so as to obtain a homogenous powdered mechanical alloy;
ii. a compaction step, wherein said homogenous powdered mechanical alloy obtained in the preceding step is compacted;
iii. a heat treatment step, wherein the compacted alloy obtained in the preceding step is heat treated; and
iv. an extrusion step, wherein the heat treated alloy obtained in the preceding step is extruded.
In accordance with a further aspect, the present invention provides a process for producing thermoelectric materials based on two or more elements selected in the group constituted by Bi, Sb, Te and Se, which process may comprise:
i. a mechanical alloying step wherein determined amounts of the elements Bi, Sb, Te or Se are mixed so as to obtain a homogenous powdered mechanical alloy;
ii. a compaction step, wherein said homogenous powdered mechanical alloy obtained in the preceding step is compacted;
iii. a heat treatment step, wherein the compacted alloy obtained in the preceding step is heat treated;
iv. an extrusion step, wherein the heat treated alloy obtained in the preceding step is extruded; and
v. an annealing step wherein the extrudate obtained in the preceding step is heated.
In accordance with yet a further aspect, the present invention relates to a process for producing thermoelectric materials based on two or more elements selected in the group constituted by Bi, Sb, Te and Se, which process may comprise:
i. a mechanical alloying step wherein determined amounts of the elements Bi, Sb, Te or Se are mixed so as to obtain a homogenous powdered mechanical alloy;
ii. an extrusion step, wherein said homogenous powdered mechanical alloy obtained in the preceding step is extruded; and
iii. an annealing step wherein the extrudate obtained in the preceding step is heated.
In accordance with an additional aspect, the present invention provides a process for producing thermoelectric materials based on two or more elements selected in the group constituted by Bi, Sb, Te and Se, which process may comprise:
i. a mechanical alloying step wherein determined amounts of the elements Bi, Sb, Te or Se are mixed so as to obtain a homogenous powdered mechanical alloy;
ii. a heat treatment step, wherein said homogenous powdered mechanical alloy obtained in the preceding step is heat treated;
iii. an extrusion step, wherein the heat treated alloy obtained in the preceding step is extruded; and
iv. an annealing step wherein the extrudate obtained in the preceding step is heated.
In accordance with yet another aspect, the present invention relates to a process for producing thermoelectric materials based on two or more elements selected in the group constituted by Bi, Sb, Te and Se, which process may comprise:
i. a mechanical alloying step wherein determined amounts of the elements Bi, Sb, Te or Se are mixed so as to obtain a homogenous powdered mechanical alloy;
ii. a heat treatment step, wherein said homogenous powdered mechanical alloy obtained in the preceding step is heat treated;
iii. an extrusion step, wherein the heat treated alloy obtained in the preceding step is extruded.
In accordance with yet another aspect, the present invention relates to a process for producing thermoelectric materials based on two or more elements selected in the group constituted by Bi, Sb, Te and Se, which process may comprise:
i. a mechanical alloying step wherein determined amounts of the elements Bi, Sb, Te or Se are mixed so as to obtain a homogenous powdered mechanical alloy;
ii. a compaction step, wherein said homogenous powdered mechanical alloy obtained in the preceding step is compacted;
iii. an extrusion step, wherein the compacted alloy obtained in the preceding step is extruded;
iii. an annealing step wherein the alloy obtained in the preceding step is heated.
In accordance with an additional aspect, the present invention relates to a process for producing thermoelectric materials based on two or more elements selected in the group constituted by Bi, Sb, Te and Se, which process comprises:
i. a mechanical alloying step wherein determined amounts of the elements Bi, Sb, Te or Se are mixed so as to obtain a homogenous powdered mechanical alloy;
ii. a compaction step, wherein said homogenous powdered mechanical alloy obtained in the preceding step is compacted;
iii. an annealing step wherein the compacted alloy obtained in the preceding step is heated.
In accordance with a further aspect, the present invention provides a mechanical alloy based on two or more elements selected in the group constituted by Bi, Sb, Te and Se. In accordance with the present invention, the mechanical alloy may be obtainable by anyone of the processes according to the invention.
In accordance with yet a further aspect, the present invention provides the use of a mechanical alloy according to the invention, as cooler, as stabilizer in a electronic device and as power generator.
In accordance with yet an additional aspect the present invention provides a mechanical alloy based on two or more elements selected in the group constituted by Bi, Sb, Te and Se, said mechanical alloy having a figure of merit Z greater than 2.65xc3x9710xe2x88x923 Kxe2x88x921 (for example, for type n) and greater than 2.9xc3x9710xe2x88x923 Kxe2x88x921 (for example, for type p) and a compression strength of at least 900 kg/cm2.